This study, responding to the increasing number of teachers traveling between EFL and ESL contexts in the global world, investigated one American teacher's experiences in teaching Chinese learners of English across geographical contexts (China and the U.S.). Specifically, the teacher taught various EFL courses in Chinese universities and ESL composition classes to international Chinese students enrolled in the U.S. Drawing upon the concept of transnationalism and teaching expertise, this study explored the contextual factors influencing the teacher's development of expertise in teaching and his responses to the contextual factors in his teaching practice. Implications for the design of TESOL teacher education are also provided.